The objectives of the current study were to construct a colorectal cancer-specific quality of life (QL) questionnaire module to be used in conjunction with the European Organization for Research and Treatment of Cancer (EORTC) QLQ-C30 and to test its reliability and validity in The Netherlands. Module construction took place following the EORTC guidelines for module development. The module--the QLQ-CR38--consists of 38 items covering symptoms and side-effects related to different treatment modalities, body image, sexuality and future perspective. This module was tested among 117 colorectal cancer patients on several occasions. The timing was prior to treatment with radiotherapy or chemotherapy, during treatment and 3 months following the second assessment. For purposes of test-retest reliability, a subsample of patients completed the QLQ-CR38 1 week following the third assessment. Multitrait scaling analysis confirmed the hypothesised scale structure of the function scales but not of the symptom scales. Cronbach's alpha coefficients for seven of the nine scales exceeded the 0.70 criterion at one or both assessments. The test-retest reliability for all scales and one single item was 0.78 or higher. The stability of the two remaining single items was lower. On the basis of known-groups comparisons, selective scales distinguished clearly between patients differing in disease stage, initial and on-treatment performance status and the presence of a stoma. Additionally, selective scales detected change over time as a function of change in performance status and treatment-induced change. These results lend support to the clinical validity of the QLQ-CR38 as a supplementary questionnaire for assessing specific QL issues relevant to patients with colorectal cancer. Additional efforts to test the module's cross-cultural validity are needed.